1. Field of the Invention
The present invention relates to a composition for use in manufacturing a fiber-reinforced molded ceramic body, a method of manufacturing a fiber-reinforced molded ceramic body from such a composition, and a method of producing a ceramic body according to slip casting.
2. Description of the Related Art
One typical process of manufacturing molded ceramic bodies is known as a slip casting process. According to the slip casting process, a ceramic slurry or slip which comprises a suspension of ceramic power in water is poured into a mold made of a water-absorbing material such as plaster or the like, and the water of the slurry is absorbed into and passed through the mold, thereby forming a molded body (green body) of ceramic powder. The molded green body is then fired into a final sintered ceramic product. The slip casting process is generally suitable for manufacturing molded ceramic bodies of complex shapes.
In the production of a molded ceramic body according to the slip casting process, it has been the general practice to add, to the slip, an organic binder such as of carboxy-methylcellulose, methylcellulose, polyvinyl alcohol, or the like for increasing the dry mechanical strength of the molded ceramic body. However, since the added organic binder increases the shrinkage ratio of the molded ceramic body, the molded ceramic body is liable to crack when removed from the plaster mold and dried. Another problem imposed by the use of an organic binder is that the process of producing a final ceramic component from the molded ceramic body is time-consuming because the molded ceramic body has to be degreased before being fired.
It has also been customary to add fibrous ceramic, rather than an organic binder, to a ceramic slip before it is poured into a mold. The addition of only one type of fibrous ceramic in a certain range is not effective because the molded ceramic body has a relatively low mechanical strength and a relatively high shrinkage ratio, making the molded ceramic body liable to crack when removed from the plaster mold and dried.
According to one specific slip casting process, a ceramic slurry or slip is prepared by adding water and a dispersant to a ceramic mixture of silicon nitride (Si.sub.3 N.sub.4), yt-trium oxide (Y.sub.2 O.sub.3), aluminum oxide (Al.sub.2 O.sub.3), and spinel (MgAl.sub.2 O.sub.4), and the prepared slip is poured into a porous mold made of plaster. The slip is then molded under a low pressure in the mold, thereby producing a preliminary molded body. The preliminary molded body is thereafter dried into a molded ceramic body. This slip casting process suffers problems when it is employed to produce a large molded ceramic body such as an axial turbine having a diameter of 100 mm or greater. Specifically, such a large molded ceramic body is highly likely to crack a few hours after being removed from the mold if a preliminary molded body therefor is naturally dried or dried in a standard drying mode in which it is maintained at a humidity ranging from 30 to 50% and a temperature of 180.degree. C. for 15 hours in a constant-temperature, constant-humidity drier.
In order to prevent the molded ceramic body from cracking, it is necessary to dry the preliminary molded body in a high-humidity, constant-temperature drying mode immediately after it is removed from the mold. In the high-humidity, constant-temperature drying mode, the preliminary molded body is dried first at a temperature of 25.degree. C. and a humidity of 90% in the constant-temperature, constant-humidity drier, with the humidity being subsequently gradually reduced over a long period of time. After the high-humidity, constant-temperature drying mode, it is then necessary to dry the preliminary molded body in the standard drying mode. Consequently, it is very time-consuming to dry the preliminary molded body into the ceramic body.
Japanese laid-open patent publication No. 4-144952 discloses a slip casting process of molding a ceramic body using a water-retaining additive which is composed of a polysaccharide produced by a microorganism. According to the disclosed process, a slurry or slip is prepared by adding water, a dispersant, and a polysaccharide to a ceramic powder composed of silicon nitride, yttrium oxide, and spinel, and poured into a plaster mold, thereby producing a preliminary molded body in the shape of a turbine rotor. The preliminary molded body is then dried with heat into a turbine rotor.
The above publication indicates that, since the polysaccharide is added to the ceramic powder, no density difference is caused between thin and thick portions of the turbine rotor, e.g., in a vane thereof, thus preventing the preliminary molded body from being deformed when dried with heat. The publication also reveals that a slurry is prepared by adding, as such a polysaccharide, .beta.-1, 3-glucan (also known by its trivial name: curdian), represented by the chemical formula given below, in the range of from 0.3 to 5.0 parts by weight to 100 parts by weight of a ceramic powder composed of silicon nitride, yttrium oxide, and spinel. ##STR1##
However, the slip casting process disclosed in the above publication is disadvantageous in that the preliminary molded body tends to crack when dried with heat. Depending on the composition of the ceramic powder, the polysaccharide added in the disclosed range is liable to make the slurry too viscous to allow the removal of bubbles produced during the preparation thereof and to be cast into the plaster mold.